Nicholls, Mark wrote:
data Shape = Circle Int
| Rectangle Int Int
| Square Int
Isn't this now "closed"...i.e. the statement is effectively defining
that shape is this and only ever this....i.e. can I in another module
add new "types" of Shape?
Yes, but in most cases, this is actually a good thing. For instance, you
can now define equality of two shapes:
equal :: Shape -> Shape -> Bool
equal (Circle x) (Circle y) = x == y
equal (Rectangle x1 x2) (Rectangle y1 y2) = x1 == x2 && y1 == y2
equal (Square x) (Square y) = x == y
In general, the "open" approach is limited to functions of the form
Shape -> ... -> Shape / Int / something else
with no Shape occurring in the other ... arguments.
I'm trying to teach myself Haskell....I've spent a few hours going
through a few tutorials....and I sort of get the basics...
[...]
After many years of OOP though my brain is wired up to construct
software in that 'pattern'....a problem for me at the moment is I cannot
see how to construct programs in an OO style in Haskell....I know this
is probably not the way to approach it...but I feel I need to master the
syntax before the paradigm.
This approach is probably harder than it could be, you'll have a much
easier time learning it from a paper-textbook like
http://www.cs.nott.ac.uk/~gmh/book.html
http://web.comlab.ox.ac.uk/oucl/publications/books/functional/
http://haskell.org/soe/
After all, it's like learning programming anew.
Regards,
apfelmus
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